Abstract

Stratigraphic and structural relations exposed in a 90-m-long trench across the Reelfoot scarp in the central New Madrid Seismic Zone (NMSZ) provide data to assess the style and timing of late Holocene tectonic surficial deformation in the central NMSZ. Near-surface deposits exposed in the trench include natural levee, overbank, colluvial, and liquefaction-related deposits. The levee deposits consist of fine-grained, cross-bedded sands and silty sands and are overlain by clayey overbank and scarp-derived colluvial deposits. Liquefaction-related features include sand dikes and sills that intrude into the levee and overbank deposits, and a possible older extrusive sand deposit. Charcoal and archaeological artifacts from a deposit inset into and overlying the levee deposits suggest that the levee deposits are older than about A.D. 800 to 900. Charcoal from the overbank deposits yielded an age estimate of cal. A.D. 1310 ± 90; charcoal from the overlying colluvial deposits yielded an age estimate of cal. A.D. 1540 ± 90.

Distinct marker beds within the levee deposits define a broad monocline that parallels the ground surface and exhibits more than 5 m of down-to-the-east vertical separation. This fold consists, in part, of four smaller-scale flexures each having amplitudes of about 1 m. Associated with these flexures are numerous west-dipping normal faults that have a total net vertical separation of about 0.4 m in a down-to-the-west sense, which is opposite in sense to that exhibited by the scarp and the silty marker beds. We interpret that these faults are related to extension in the crest of the monocline, and that the monocline represents deformation above a west-dipping reverse fault that reaches or approaches the ground surface east of the trench and the base of the scarp. At the trench site, this interpretation places the surface projection of the fault near the western margin of Reelfoot Lake.

Stratigraphic relations exposed in the trench and several shallow boreholes permit identification of at least one and possibly three late Holocene earthquakes. Stratigraphic relations and age-estimates best support the interpretation that the unweathered liquefaction related features exposed in the trench are a result of the 1811–12 earthquakes, and that the scarp-derived colluvial deposits are a result of a prior event. Radiocarbon analyses show that this penultimate event occurred between about A.D. 1310 and A.D. 1540. Stratigraphic evidence of a third event prior to about A.D. 900 is present but equivocal. Given that the most-recent event occurred in A.D. 1812, we estimate that the time between the two most-recent earthquakes large enough to produce liquefaction and/or surface deformation was about 200 to 600 years.

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